Four anesthesia providers (operators) participated in the study: The first operator performed CP in all patients with his back towards the video monitor; the second operator performed laryngoscopy using GVL and GT insertion trials and he was not aware about the nature of study ; the third operator assessed the effectiveness of the applied CP and determined the position of the esophageal entrance in relation to the glottis. The fourth operator, who was standing behind the second operator, signaled to the second operator to start attempt for each trial with the predetermined CP forces and to the first operator who applying CP to stop or continue accordingly.